Many of you are probably aware of the "joule thief" circuit. If you aren't, it's a very simple voltage booster that is normally used to power an LED off of a mostly dead AA battery, but it can be used for other applications where a voltage boost is needed. One such application is charging a battery off of a battery with a smaller volatge rating. There are designs out there for battery chargers using a 9 volt battery and then step down the voltage to the level of the battery to be charged. But this circuit uses any single battery with a rating of 1.5 volts, such as a AA, C, or D battery, and bumps the voltage up to a level that can charge a battery with a voltage rating of 5 volts or lower. This curcuit is extremely simple, dirt cheap, and can be built in an afternoon using parts taken from other old devices.
Step 1: Parts and Pieces
Parts for the charger include:
2N3904 NPN transistor
1K ohm resistor
Toroid transformer core
SPST Switch (not pictured)
A word about some of the parts:
The toroid core can be any size, but nothing fancy or big is needed. A small toroid is probably better just because it is more compact. Only one toroid is used in the circuit, but two are pictured for size reference.
The diode can be just about any diode, but the lower the forward voltage drop, the better. Germanium diodes work the best. To find the forward voltage drop, simply hook a diode up to a battery, and measure the difference in voltage with and without the diode in the circuit. If the voltage drop is about equal to the voltage being supplied, make sure the diode isn't hooked up backwards.